The arrival at Baker Street of a client who, unusually, seeks advice from both men brings to their attention the existence of an isolated manor house in the east of England known as Blackstone Grange. The place, for so long the locus of wild rumours of witchcraft and sorcery, has acquired a very modern new master – a charismatic socialite named ‘Honest’ Jim Sheedy, a man of boundless ambition whose influence over British society appears, inexplicably, to be growing.

The detective learns that Colonel Sebastian Moran, released early from prison, travels to meet with Sheedy on the very day of his freedom. But curiosity curdles into obsession when Mycroft warns his brother, in the strictest possible terms, against investigating any further.